Sound at frequencies below 20 Hertz is termed “infrasound.” A particularly favorable property of infrasound is its propagation over long distances with little attenuation. Infrasound has this property because atmospheric absorption is practically negligible at infrasonic frequencies, and because there is an acoustic “ceiling” in the stratosphere where a positive gradient of the speed of sound versus altitude causes reflections of infrasonic rays back to Earth. Infrasound propagation over long distances (e.g., thousands of kilometers) is predominantly due to refractive ducting from the upper layers in the atmosphere, while propagation over short distances is completed by direct path.
The density, acoustic impedance, and speed of sound through different human and animal tissues varies depending upon location of the auscultation. When an acoustic signal travels through tissue layers, the amplitude of the original signal becomes more attenuated with depth of the acoustic signal source. Attenuation (i.e. energy loss) could be due to absorption, reflection, and scattering at interfaces of different tissues. The degree of attenuation also depend upon frequency of the sound wave and the distance it travels. Generally speaking, a high frequency acoustic signal is associated with high attenuation thus limiting tissue penetration, but lower frequencies do not have attenuation issue thus providing physicians better understanding of the heart performance. More than 60% power spectral density of heart signals fall in infrasonic bandwidth. Low frequency acoustic signals detected from different human organs, such as the heart, are valuable to physicians for monitoring heart and lungs.
Microphones and stethoscopes are regularly used by physicians in detecting sounds for monitoring physiological conditions. Phonocardiography has been in use for more than 75 years to monitor heart beats as well as to detect the audible sound of the blood flowing through the heart. These physiological condition monitors are coupled directly to a person's body and processes are measured either by listening or by recording the signals in certain bandwidth. The physiological processes such as respiration and cardiac activity are reflected in a different frequency bandwidth from 1/10 Hertz to 500 Hertz. Other stethoscopes are capable of monitoring only audible frequency bandwidth, and are not capable of monitoring infrasonic frequencies below 20 Hz. Low frequency acoustic signals below 20 Hertz are not audible, but can provide useful information to physicians.
Inside of a normal heart, there are four chambers namely; the right atrium, the left atrium, the right ventricle, and the left ventricle. The function of a heart is to keep blood flowing in one-way direction. When a valve opens, the valve lets the right amount of blood through, and then closes to keep blood from flowing backwards in between beats. An easy and relatively inexpensive assessment of any patient's cardiac status can be determined by sounds in the chest. The key to good auscultation lies in low and high pitched sounds. As the heart beats, blood flows from right atrium into the right ventricle through the tricuspid valve.
Blood then flows to the lungs through pulmonary valve (sometimes also called semilunar valve) to pick up right amount of oxygen. The blood flows from the lungs back into the left atrium and enters into the left ventricle through the mitral valve. Blood then is pumped to the aorta through the aortic valve and goes out to rest of the body providing oxygen and nutrients to the body cells. All four chambers (right atrium, right ventricle, left atrium, and left ventricle) must contract at just the right time for normal heart to functioning properly. The proper timing is coordinated by heart's electrical pathways. The electrical signals are produced by the sinoatrial node (SA node) and atrioventricular node (AV node).
The SA node is a group of cells located in the right atrium that initiates contraction of both atria to push blood into their corresponding ventricles. Due to insulation between the atria and ventricles, the SA node signals do not continue directly to the ventricles but pass through the AV node, which is another group of cells located in the floor of right atrium between the atria and ventricles. The AV node regulates the signal to ensure that the atria are empty and closed before the ventricles contract to push the blood out of the heart. The SA node sends signals to stimulate the heart to beat between 60-100 times per minute.
The cardiovascular system is complex and numerous problems could take place inside anywhere from the electrical system of the heart to the large or small blood vessels. There are over 60 different types of cardiovascular disease, all of which somehow affect the cardio or vascular systems. The heart sounds generated by the beating of heart and the resultant flow of blood can provide important information about the condition of the heart. In healthy adults, two normal heart sounds occur in sequence with the heartbeat. A first sound is produced based on the closure of the atrioventricular valves (i.e. mitral and tricuspid valves) located between the atria and ventricles, and is referred to as S1. A second sound is produced as a result of closure of the semilunar valves (i.e. pulmonary and aortic valves), which control the flow of blood as it leaves the heart via the arteries, and is referred to as S2.
The first heart sound S1 consists of four sequential components: 1. Small low frequency vibrations that coincide with the beginning of left ventricular contraction. 2. High frequency vibration, easily audible related to mitral valve closure (M1). 3. A second high frequency component related to tricuspid valve closure. 4. Small frequency vibrations that coincide with the acceleration of blood into great vessel.
In addition to these normal sounds, a variety of other sounds may be present but requires highly sensitive microphone with lowest acoustic background noise level along with filters to pick up these sounds. A third low frequency sound, which may be heard at the beginning of the diastole, is referred to as S3. A fourth sound may be heard in late diastole during atrial contraction, is referred to as S4. These sounds can be associated with heart murmurs, adventitious sound, ventricular gallop and gallop rhythms. The S4 provides information about hypertension and acute myocardial infarction.
The cardiac sounds S1, S2, S3, and S4 can be attributed to specific cardiac activity. S1 is attributed to the onset of the ventricular contraction (10-140 Hertz bands). S2 is attributed to closure of the semilunar valves (10-400 Hertz bands). S3 may be attributed to ventricular gallop, which may be heard during rapid filling (i.e. diastole) of the ventricles. S4 may be attributed to atrial gallop, which may be heard in late diastole, during atrial contraction. S3 and S4 are of very low intensity and can be heard externally when amplified.
Other sounds may be heard from opening snaps of the mitral valve or ejection sound of the blood in the aorta which indicates valve malfunctions, such as stenosis or regurgitation. Other high frequency murmurs can occur between the two major heart sounds during systole or diastole. The murmurs can be innocent but can also indicate certain cardiovascular defects.
Continuous fetal heart monitoring is an important step to evaluate the well-being of a fetus. The fetal heart rate may indicate if the fetus is getting enough oxygen. Most of the time ultrasound transducers are used for monitoring fetal heart rate as conventional stethoscopes undesirably pick up signals from maternal abdominal vessels. Due to abdominal fat of the mother or fetal positioning, it may be difficult to monitor fetal heart passively, so most of the time ultrasound transducers are used where ultrasound pulses are radiated towards the fetus and reflective pulses are used for monitoring. If enough reflective signals are not received, the penetration depth of ultrasound pulses are increased which may decrease quality and signal-to-noise ratio. The high frequency ultrasound signals become attenuated due to absorption, reflection, and scattering due to abdominal fat. The infrasound signals have relatively very low attenuation coefficient hence the signals are expected to be of high quality with better signal to noise ratio and helpful to gynecologists.
Many heart sounds are in a low-frequency band spectrum with low intensity level and may require extremely sensitive infrasonic microphone to acquire useful information that cannot be perceived by the physician's ear. The passive filtering may be useful to record low and high frequency bands separately. The sounds are of short duration and highly non-stationary but enable to measuring systolic and diastolic time intervals, which may have diagnostic importance.
Accordingly, there is a need for a monitoring device that overcomes the disadvantages presented by the prior art.